Base-catalyzed Sulfurative Condensation of 2-Oxoindoles to Isoindigos Using Elemental Sulfur

Herein, the base-catalyzed formation of isoindigos is described through the cascade reaction of 2-oxoindoles with elemental sulfur (S-8) serving as an environmentally friendly and cheap sulfurating reagent. Readily available organic amines, especially 1,4-diazabicyclo[2.2.2]octane (DABCO), exhibit excellent activity for this protocol, thus simply and efficiently providing structurally diverse isoindigos in moderate to high yields with a broad substrate scope. A plausible mechanism involving the generation of 3-thioisatin intermediates and the subsequent self-condensation, was proposed on the basis of mechanistic studies. The practicality of the methodology was further demonstrated by the synthesis of gram-scalable drug molecules and value-added spirothiopyrans.